1. Field of the Invention
The present invention relates to a radio-frequency switch circuit and a semiconductor device, and more particularly, to a radio-frequency switch circuit employing a field-effect transistor, and a semiconductor device in which the radio-frequency switch circuit is integrated.
2. Description of the Background Art
In recent years, as performance of mobile communication apparatuses is improved, there is an increasing demand for small-sized and high-performance radio-frequency semiconductor devices. Particularly, low insertion loss and low distortion are simultaneously required for a radio-frequency switch circuit which performs antenna switching. Therefore, a method of constructing a radio-frequency switch circuit using a plurality of field-effect transistors (hereinafter referred to as “FETs”) connected together has been proposed.
FIG. 12 is a diagram illustrating a conventional radio-frequency switch circuit described in Japanese Patent Laid-Open Publication No. H08-23270. The conventional radio-frequency switch circuit of FIG. 12 comprises first to third input/output terminals 7 to 9, reception transfer FETs 11a and 11b, a transmission transfer FET 12, a reception shunt FET 21, transmission shunt FETs 22a and 22b, gate bias resistors 31a, 31b, 32a, 32b, 33, and 34 each of which has a resistance of R, and first and second control terminals 41 and 42. The conventional radio-frequency switch circuit is generally used while the first input/output terminal 7 is used as a reception terminal, the second input/output terminal 8 is used as a transmission terminal, and the third input/output terminal 9 is used as an antenna terminal.
In order to receive a radio-frequency signal, a high voltage is applied to the first control terminal 41 while a low voltage is applied to the second control terminal 42. Thereby, the FETs 11a, 11b, 22a, and 22b are turned ON while the FETs 12 and 21 are turned OFF, so that the first input/output terminal 7 and the third input/output terminal 9 are short-circuited. Therefore, the received signal input from the third input/output terminal 9 is output from the first input/output terminal 7.
In order to transmit a radio-frequency signal, a low voltage is applied to the first control terminal 41 while a high voltage is applied to the second control terminal 42. Thereby, the FETs 11a, 11b, 22a, and 22b are turned OFF while the FETs 12 and 21 are turned ON, so that the second input/output terminal 8 and the third input/output terminal 9 are short-circuited. Therefore, the transmission signal input from the second input/output terminal 8 is output from the third input/output terminal 9.
In the conventional radio-frequency switch circuit, the two FETs 11a and 11b are connected in series for the purpose of reception transfer, and the two FETs 22a and 22b are connected in series for transmission shunt. Therefore, during transmission, the radio-frequency signal voltage input from the second input/output terminal 8 is divided by the FETs 11a, 11b, 22a, and 22b. As a result, even when a large signal is input from the second input/output terminal 8, the FETs 11a, 11b, 22a, and 22b can easily maintain the OFF state, so that excellent distortion characteristics and high input satuation power can be achieved as compared to when only one FET is used.
In the conventional radio-frequency switch circuit, the gate bias resistors 31a, 31b, 32a, 32b, 33, and 34 are provided for the purpose of prevention of leakage of the radio-frequency signal. Connection to the gate electrode of each of the two series-connected FETs of FIG. 12 requires the resistance R of about 40 to 50 kΩ in order to prevent degradation of performance due to signal leakage. The same applies to a radio-frequency switch circuit of FIG. 13 in which two or more FETs are connected in series, i.e., a plurality of resistors having the same resistance R are used to construct the radio-frequency switch circuit of FIG. 13.
In the conventional radio-frequency switch circuits of FIGS. 12 and 13, resistors having the same value R of as large as about 40 to 50 kΩ are used for all gate bias resistors connected to the gates of FETs. However, it is difficult to use a material having a large sheet resistance in a semiconductor process. Therefore, when resistance elements of about 40 to 50 kΩ are formed on a semiconductor substrate, the resistance elements occupy a large area on the substrate. Therefore, the chip size of the conventional radio-frequency switch circuit increases, resulting in high circuit cost.